append iso raw

append edd iso bigraw

root@mfsbsd:~ # fetch http://10.10.1.1/images/microsoft/WinPE_Strelec_2020.05.31.iso > /dev/null
WinPE_Strelec_2020.05.31.iso                          4285 MB   22 MBps 03m09s

Syslinux memdisk is a 32-bit application and relies on old BIOS calls to virtualize the ISO in RAM. I think you are hitting some kind of limitation due to 32-bit or the BIOS. Better to ask this question in irc #syslinux or syslinux mailing-list.